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Abstract

Re-wetting river floodplains as part of hydrological restoration can provide environmental, economic and social benefits. However, labile forms of nutrients have accumulated in many floodplains and enhanced nutrient mobility has been reported after re-wetting of some floodplain sediments. Increases in nutrient mobility could restrict the net benefit of floodplain restoration, although the long-term impacts of restoration on nutrient mobility are largely unknown. We used in situ hydrological and hydrochemical monitoring to examine the mobility of phosphorus (P) during a range of hydrological events within a long-restored (more than 20 years) floodplain in Norfolk, England. Our data suggest that P was released from sediments to porewater and surface water following inundation of the floodplain. The concentration of P exceeded 6.5 mg P L−1 in porewater and 0.5 mg P L−1 in surface water within six weeks of inundation, significantly higher than the concentration in rainwater (<0.015 mg P L−1) or in floodwater from the adjacent river (¯x = 0.11mgPL−1) that inundated the floodplain. Sediment geochemistry was used to predict changes in the mobility of P following inundation, based on the molar ratio of redox sensitive P to iron or the molar quantity of redox-sensitive P within the floodplain sediments. We found that short-term hydrological events, both external (tidal pulse) and internal (water-table drawdown) in origin, were associated with increases in the concentration of dissolved forms of P within the floodplain drainage network. These increases coincided with porewater movement to the drainage network, suggesting that advection along shallow sub-surface flowpaths was important for the transport of P. Our research indicates that some restored floodplains could inadvertently represent sources of P to adjacent streams and rivers.